The present invention relates to a high purity tantalum sputtering target comprising a uniform structure and which enables uniform sputtering at a high deposition rate in a stable manner.
In recent years, the sputtering method for forming films from metals, ceramics and other materials has been used in numerous fields such as electronics, corrosion resistant materials and ornaments, catalysts, as well as in the manufacture of cutting/polishing materials and abrasion resistant materials.
While the sputtering method itself is a well-known method in the foregoing fields, recently, particularly in the electronics field, a tantalum sputtering target suitable for forming films of complex shapes, forming circuits or forming barrier films is in demand.
Generally speaking, this tantalum target is manufactured by repeating the forging and annealing (heat treatment) of an ingot or billet formed by performing electron beam melting and casting to a tantalum raw material, and thereafter performing rolling and finish processing (machining, polishing, etc.) thereto in order to process the ingot or billet into a target.
In this kind of production process, the forging performed to the ingot or billet will destroy the cast structure, disperse or eliminate the pores and segregations, and, by further performing annealing thereto, recrystallization will occur, and the densification and strength of the structure are improved to obtain the target.
The molten and cast ingot or billet generally has a primary crystal grain size of 50 mm or more. As a result of subjecting the ingot or billet to forging and recrystallization annealing, the cast structure is destroyed, and generally uniform and fine (100 μm or less) crystal grains can be obtained.
Meanwhile, when sputtering is performed using a target produced as described above, it is said that the recrystallized structure of the target becomes even finer and more uniform, more uniform deposition is possible with a target in which the crystal orientation is aligned toward a specific direction, and a film with low generation of arcing and particles and stable characteristics can be obtained. Thus, measures are being taken for achieving a finer and more uniform recrystallized structure and aligning the crystal orientation toward a specific direction in the production process of the target (for example, refer to Patent Document 1 and Patent Document 2).
Moreover, disclosed is a high purity Ta target for forming a TaN film to be used as a barrier layer against a Cu wiring which is obtained by containing 0.001 to 20 ppm of an element selected among Ag, Au and Cu as an element having self-sustained discharge characteristics, causing the total amount of Fe, Ni, Cr, Si, Al, Na, and K as impurity elements to be 100 ppm or less, and using a high purity Ta in which the value obtained by subtracting such impurity elements is within the range of 99.99 to 99.999% (refer to Patent Document 3).
Nevertheless, when reviewing these Patent Documents, there is no disclosure to the effect that the inclusion of specific elements realizes a finer structure and thereby stabilizes the plasma.
Moreover, since the wiring width is becoming even narrower with the state-of-the-art semiconductor devices in recent years, Ta or TaN films that are used as the barrier layer of a Cu wiring are demanded of having an even thinner film thickness (ultra-thin film) while maintaining barrier characteristics that are equal to, or better than, conventional technologies.
In order to meet the foregoing demands, desired is a high purity material from which impurities, which inhibit the adhesion of the film, are reduced as much as possible, but a Ta material having a purity of 6N had a drawback in that it is difficult to obtain a fine and uniform structure since crystal grains would grow easily when forging is performed for refining the crystal grain size or when recrystallization is performed via heat treatment to the strain from the plastic working process that is introduced in the rolling process. Furthermore, when a target having this kind of coarse crystal structure is sputtered, there is a problem in that the uniformity of the deposited film will deteriorate.
In light of the above, the present applicant previously filed an application for a tantalum sputtering target containing tungsten and other elements as essential components in an amount of 1 massppm or more and 100 massppm or less, and having a purity or 99.9985% or higher as a result of discovering that, by adding trace amounts of components that have a limited effect on the film characteristics, it is possible to achieve a fine and uniform crystal grain size while maintaining a high purity (refer to Patent Documents 4 to 6).
Nevertheless, when the foregoing targets are used; that is, when a target having an ultra-fine crystal structure caused by excessive refinement is sputtered, in certain cases major problems were indicated to the effect that the deposition rate becomes slow and high-speed sputtering cannot be performed in a stable and uniform manner, and the productivity would considerably deteriorate.